The protection of electric and electronic devices against interfering radiation and electric/magnetic fields requires serious attention, in order to prevent system failure and/or unsafe operating conditions. Typically metal shielding is used, in the protection of units which include cables and wires, to block interferences induced due to external fields, and to protect electric devices from interferences that may be induced by adjacently located electric cables.
Metal shielding attenuates the electromagnetic waves energy and thereby reduces energy absorption by the electrically conducting media, and emission of electromagnetic waves energy at the interface between the conductors and shielding medias. It is desirable that the shielding material provide maximal protection and attenuation of electromagnetic field noises/interferences. However, in practice, typical configurations of cable shielding employed nowadays suffer from various disadvantages, such as significant increase of the cables price and of their installation costs.
Conventional shielded cables arrangements are typically vulnerable to magnetic and electrostatic fields generated due to occurrence of lightning discharge and due to the increase of the Earth potential in the grounding area at the location of lighting hit. These phenomena may occur across industrial networks and power circuits and induce voltages that are dangerous for the electrical/electronic equipment electrically fed from the networks/circuitries.
Aerial power transmission lines, contact wires of AC (alternating current) operated trains, radio stations etc., create magnetic fields which induce voltages and currents in cables installed adjacent to them. Under certain conditions the levels of these induced voltages and currents endanger the cable insulation and the devices to which the cable is connected.
Conventional power cables and wires are designed to attenuate the surface effect influence (i.e., to minimize skin effects losses) by using cables' cores formed as a bundle of thin conductors, which are twisted in order to reduce the magnetic field outside the cable (i.e., the field generated by the cable) and obtain reduction coefficient factors in a range of 20 to 30 at a distance of 0.2 m from the center of the power cable. However, twisting the conductors also yields an increase in the in the length of the cable conductors, and consequently increases the active resistance of the conductors. Moreover, the twisting of the conductors may also be inefficient in preventing the losses caused by the surface effect.
Various designs and properties of single and three-phase cables are described in U.S. Pat. No. 6,506,971. This patent describes method of designing a single- or a multi-phase electric cable for conducting current through insulated conductors and creating a weak external magnetic field, so as to obtain a cable wherein at least one of the above-mentioned conductors is assembled from two or more insulated sub-conductors connected in parallel, and wherein the sum of cross-sectional areas of the sub-conductors is equal to a design cross-sectional area of the conductor. The arrangement arrangements in the cable is such that each of the sub-conductors is adjacent to a conductor, or a sub-conductor, associated with either a different phase or a different current direction.